1. Field of the Invention
The present invention relates to multicolor display elements.
2. Description of the Related Art
Recently, electric papers are commercially expected as new reading and writing media in place of conventional papers. As for the electric papers, various properties or items are demanded such as higher white reflectance, higher contrast ratio, highly fine and precise images, relatively thin, lightweight, and inexpensive; and also the electric papers are of reflective type, the displayed information can be memorized, and the electric papers can be driven at lower voltages. In particular, the white reflectance and the contrast ratio are required to be as high as those of conventional papers.
Previously, reflective liquid crystals, electrophoretic elements, toner migration elements, and the like have been proposed as the candidate of electric paper. However, these proposed devices suffer from lower white reflectance. Moreover, color filters are necessary in order to provide multicolor displays from these display devices. When color filters are employed, the reflective light quantity is remarkably reduced, since color filters themselves absorb light and also one pixel should be divided into three parts of red, green, and blue, thus resulting in significantly poor visuality.
There exist a phenomenon of so-called electrocromism in which a color of a material is changed reversibly by reaction of electric oxidation and/or electric reduction. Electrochromic compounds, capable of undergoing such electrocromism, are expected to be useful for electric papers since electrochromic display elements based on the coloring and decoloring of electrochromic compounds can provide a reflective display, the displayed information can be memorized, and the electrochromic display elements can be driven at lower voltages. Further, the electrochromic display elements are expected as multicolor display elements since various colors can be displayed corresponding to the material configurations or constitutions.
As for multicolor displays utilizing electrochromic multicolor display elements in the prior art, Japanese Patent Application Publication No. 01-39086 discloses a multicolor display element in which plural species of electrochromic compounds are attached to a polymer. More specifically, in the disclosure of Japanese Patent Application Publication No. 01-39086, an electrochromic compound capable of coloring by an oxidation reaction and an electrochromic compound capable of coloring by a reduction reaction are attached to a polymer. In this configuration, one of the electrochromic compounds can be colored; however, both of the electrochromic compounds cannot be colored simultaneously.
Japanese Patent Application Laid-Open No. 10-161161 discloses a multicolor display formed by combining a specific electrochromic composition and an electrophoretic effect. The disclosed display can provide three primary colors, however, is not suited to display full color since colors cannot be mixed by way of laminating layers.
Japanese Patent Application Laid-Open No. 2003-121883 discloses a display element in which plural species of electrochromic compositions are laminated in order of the threshold voltages corresponding to the electrochromic colorings from higher or lower. However, the element is not adapted to full color since respective electrochromic compositions cannot be individually colored by merely adjusting the threshold voltages. Further, Japanese Patent Application Laid-Open No. 2003-121883 discloses no specific examples concerning the display element.